A first known type of delay apparatus consists in placing optical switches and calibrated delay lines in cascade and in alternation. The path travelled by a wave applied to the inlet of the apparatus is a function of the commands applied to the optical switches. Unfortunately, that system suffers from the drawback of imparting interference noise.
Another type of delay apparatus consists in placing a tunable wavelength converter, an optical demultiplexer, and an optical multiplexer in cascade, the outlets of the demultiplexer being coupled to the corresponding inlets of the multiplexer via delay lines all having different lengths. Thus, as a function of its wavelength, the wave delivered by the converter is directed to a single one of the delay lines.
That solution offers the advantage of being simple, but it is difficult to implement if very short switching times are desired. Moreover, means need to be provided for dynamically tuning the carrier of the converter.